In general, a metal-air battery wherein an aqueous solution is used for the electrolytic solution is a battery combining an oxidation-reduction reaction of a metal, which is a negative electrode reaction, with an oxygen reduction reaction and an oxygen generation reaction, which are a positive electrode reaction, and since oxygen as a positive electrode active material can be taken from the air and the space for holding an active material in the positive electrode can be omitted, this battery is expected as a battery having a high energy density.
For example, in the case where the negative electrode metal is zinc, a discharge reaction between two electrodes proceeds as follows.Positive electrode: O2+2H2O+4e−→4OH−Negative electrode: 2Zn+4OH−→2ZnO+2H2O+4e−
The reverse reaction is a charging reaction, and the metal-air battery functions as a primary battery and also as a secondary battery.
General configurations of a metal-air battery are illustrated in FIGS. 1 and 2. The metal-air battery is equipped with a negative electrode 1, a separator (or an electrolyte membrane) 2, a positive electrode 3, a catalyst layer 4, a porous layer 5, an aqueous electrolyte solution 6, a negative electrode collector 7, and a positive electrode collector 8. The metal-air battery has a configuration where the negative electrode 1 and the positive electrode 3 are separated by the separator 2 to prevent physical contact therebetween. The separator 2 is formed of a porous material to allow permeation of an aqueous electrolyte solution 6 that is strong alkali, or a polymer membrane capable of conducting OH− ion and has a configuration in which the aqueous electrolyte solution 6 has penetrated into pores of the negative electrode 1 and the positive electrode 3 and OH− can transfer between two electrodes during charge/discharge through the aqueous electrolyte solution. Among others, in the material of the positive electrode 3, a carbon material having electron conductivity is often used as the main component so as to ensure electron conductivity, and the outside thereof (the side opposite the separator) is opened to the atmosphere or exposed to a channel for supplying air, so that the air necessary for the discharge reaction at the positive electrode can diffuse into the positive electrode.
Furthermore, a catalyst is contained in the positive electrode to reduce the overvoltage for positive electrode reaction. The paths of electron, air and aqueous electrolyte solution must be connected to the catalyst so that the air (oxygen), water, electron and OH− participating in the electrode reaction can be delivered/received in the catalyst. Since an aqueous electrolyte solution forms the supply path for water or OH and the aqueous electrolyte solution must penetrate into the catalyst layer 4 from the separator side, the positive electrode 3 at least on the side facing the separator needs to be hydrophilic and porous. On the other hand, the surface on the opposite side of the positive electrode has a configuration of being opened to air but in order to make the configuration to allow for diffusion of air while avoiding the aqueous electrolyte solution as a strong alkali from leaking out of the open-to-air side, the air electrode on the open-to-air side is required to be hydrophobic and porous.
For satisfying these structural requirements, it is common to employ a two-layer configuration of, in order from the separator-facing side (negative electrode-facing side), a hydrophilic porous catalyst layer 4 containing a carbon material and a catalyst element as main components, and a hydrophobic porous layer 5. The current collector 8 on the positive electrode side for delivering/receiving an electron to/from an external circuit generally uses a metal mesh and is disposed to contact with the open-to-air side of the hydrophobic porous layer (FIG. 2) or disposed between the catalyst layer and the hydrophobic porous layer (FIG. 1).
In order for an efficient electrode reaction to proceed in the positive electrode having the above-mentioned configuration, it is important to promote mass transfer by ensuring a path through which a substance necessary for the electrode reaction transfers to the catalyst contained in the electrode. From the more microscopic viewpoint, a catalyst as a reaction site must be present as much as possible in a triple phase boundary that is a junction of a carbon material network capable of making electrical conduction with an external circuit and supplying an electron necessary for the reaction, an air diffusion network formed by continuous pores allowing for diffusion of air from the atmosphere outside the battery, and an aqueous electrolyte solution network capable of delivering/receiving OH− to/from the negative electrode according to charge/discharge.
Some methods have been heretofore proposed to form such a triple phase boundary. In a general two-layer configuration combining a hydrophilic catalyst layer and a hydrophobic porous layer, when an aqueous electrolyte solution penetrates into a porous catalyst layer formed of a hydrophilic material, the aqueous electrolyte solution penetrates into all pores formed in the catalyst layer, and the triple phase boundary where the aqueous electrolyte solution and air are present is substantially limited to an interface 9 between the hydrophilic catalyst layer and the hydrophobic porous layer (FIG. 3). That is, most of catalyst elements contained in the catalyst layer are surrounded by the aqueous electrolyte solution, and the air (oxygen) necessary for an electrode reaction is not supplied and cannot contribute to the reaction.
The technique employed widely in general to avoid such a situation includes a method of compounding PTFE as a water-repellent component with a catalyst layer formed of a hydrophilic carbon material. As a similar method, Patent Document 1 has proposed a method of compounding wax as a hydrophobic component with a catalyst layer. The method of compounding the hydrophobic PTFE or wax with a catalyst layer aims to form a hydrophobic portion in part of a hydrophilic porous catalyst layer, thereby forming an air (oxygen) diffusion path in the catalyst layer and increasing a triple phase boundary.
As another method, Patent Document 2 has proposed an electrode in which texture processing is applied to an interface between a hydrophilic porous catalyst layer and a hydrophobic porous layer and an uneven shape is thereby imparted to the interface. This method aims to increase the area of an interface between a hydrophilic porous catalyst layer filled with an aqueous electrolyte solution and a hydrophobic porous layer and thereby increase a triple phase boundary that is formed in the interface.
As still another method, Patent Document 3 has proposed a method of forming a porous catalyst layer from a mixture of a hydrophilic porous material having supported thereon a catalyst element, and a hydrophobic porous material. As with the method of compounding PTFE or wax, the basic idea of this method aims to compound a hydrophobic material in a hydrophilic porous catalyst layer, thereby forming an air diffusion path, increasing a triple phase boundary, and achieving efficiency of the electrode reaction.
On the other hand, similarly to the metal-air battery, control of hydrophilic and hydrophobic materials is performed also in the catalyst layer of an air electrode of a fuel cell in which the active material is oxygen and an oxygen reducing catalyst is disposed. In the catalyst layer of an air electrode (positive electrode) of a fuel cell, it is important to realize a high density of the catalyst present in a triple phase boundary formed by continuous pores enabling oxygen as an active material in air to diffuse, an electrolyte material as a proton conducting path, and a carbon material as an electron conducting path. In general, the electrolyte material as a proton conducting path shows minimum proton conduction resistance under highly humid conditions and therefore, the material constituting the catalyst layer must be made hydrophilic to thereby keep the electrolyte material in a wet state and suppress increase of the proton conduction resistance.
On the other hand, in order to prevent blockage of a gas diffusion path due to water formed by an air electrode reaction, a diffusion path for air in which oxygen as an active material is contained needs to be ensured by compounding a hydrophobic material in a catalyst layer. High-level control of hydrophilic and hydrophobic materials satisfying these two contradictory requirements is required particularly for a high-performance catalyst layer generating power at a high current density. From such viewpoint, Patent Document 4 has proposed a method where carbon black being hydrophobic and having a three-dimensional structure advantageous to gas diffusion is incorporated into a catalyst layer. Furthermore, in Patent Document 5, as a technique for satisfying both conditions wherein an electrolyte material is kept in a wet state so as not to impair proton conductivity of an electrolyte material as a proton conduction path and wherein a gas diffusion path is ensured, a technique of dispersing a hydrophobic carbon material having an agglomerate configuration in a catalyst layer has been proposed.
Patent Document 6 discloses an electrode for a polymer electrolyte fuel cell, in which a carbon material having a water vapor adsorption amount of 1 to 20 ml/g at a relative humidity of 90% is used as a gas diffusion carbon material, whereby blockage of a gas diffusion path due to water can be effectively prevented and a current can be generated with stable voltage.
Patent Document 7 discloses a technique for enhancing the gas diffusibility in an electrode by satisfying the condition of (D90/D50)≥2.5, wherein D50 and D90 are particle diameters when the volume cumulative frequency in the particle size distribution of a catalyst powder for a fuel cell reaches 50% and 90%, respectively.
Patent Document 8 discloses an electrode composed of a first layer containing a mesoporous nano-structured hydrophobic material and a second layer being disposed on the first layer and containing a mesoporous nano-structured hydrophilic material, so as to provide a material suitable for use as a gas diffusion electrode.